Digital communication systems handle transmission errors by providing redundancy information that allows for error detection and/or error correction. To facilitate error detection, redundancy check information, such as cyclic redundancy check (CRC) bits, is transmitted along with user data. A receiver uses the redundancy check information to determine if an error occurred in the transmission. Error correction generally requires more redundancy. Complex forward error correction coding, such as convolutional turbo coding or CTC, may provide enough redundancy for the receiver to correct most transmission errors. Alternatively, the receiver can simply request retransmission of a data packet if the data packet was received with error. This scheme is referred to as automatic repeat request (ARQ). More often, however, a combination of partial forward error correction coding and repeated transmission proves to be more efficient, and is referred to as a hybrid ARQ (HARQ) scheme. In particular, a first transmission includes the user data along with only a portion of the forward error correction information. When the receiver receives the transmission, the receiver attempts to correct transmission errors. If the attempt fails, the receiver requests a retransmission. The transmitter can either repeat the same transmission or transmit a different portion of the redundancy information along with the same or different portion of the user data in subsequent retransmissions. The former is referred to as chase combining (CC) HARQ, and the latter incremental redundancy (IR) HARQ. Hereinafter, the term “retransmission” refers to a requested retransmission of data in an HARQ scheme, wherein the first transmission of the data was received with uncorrectable error. The bits in a retransmission are not necessarily identical to those in the previous transmission, such as in IR HARQ.
Even though the repeated transmissions in IR HARQ differ from one to the next, some of the data are repeated. If the channel quality does not change, the same data likely will suffer the same channel distortion. Methods have been proposed to achieve diversity in channel quality to improve the performance of HARQ. One particular method, referred to as constellation rearrangement, alternates modulation schemes on retransmissions. For example, in a proposal to the 3GPP TSG-RAN Working Group 1 Meeting #19, document number TSGR1#19(01)0237, Panasonic proposed a method of enhancing HARQ by rearranging the constellation. This proposal is briefly described below.
A constellation diagram represents a digital quadrature amplitude modulation (QAM) or phase-shift keying (PSK) scheme. In QAM, the data bits modulate two orthogonal carrier waves, i.e., a cosine wave, or an in-phase carrier, and a sine wave, or a quadrature carrier, which are combined and transmitted. The data bits determine the amplitudes and phases of the carrier waves, although in QAM the phases of the carrier waves are either 0° or 180°. In PSK, the data bits modulate the phase of a single carrier wave. FIG. 1 illustrates an exemplary constellation diagram for an order-4 QAM (16-QAM) scheme, in which a modulation symbol has 16 possible values and thus represents 4 bits of data
In FIG. 1, the I-axis represents the amplitude of the in-phase carrier, and the Q-axis represents the amplitude of the quadrature carrier. The data bits are grouped into symbols each including 4 bits, i1q1i2q2. i1 and i2 modulate the in-phase carrier, where i1 modulates the phase of the in-phase carrier and i2 modulates the amplitude of the in-phase carrier. In particular, the phase is 0° if i1=0 and 180° if i1=1, and the amplitude is greater when i2=1 than that when i2=0. Similarly, q1 and q2 modulate the quadrature carrier, where q1 modulates the phase of the quadrature carrier and q2 modulates the amplitude of the quadrature carrier.
Because the data bits modulate the carrier waves in different aspects, the data bits have different significances and different reliabilities. Particularly, absolute values are more difficult to detect with accuracy than the sign of the amplitude (or the phase) of the carrier wave. In other words, in the example given in FIG. 1, bits i2 and q2 are more susceptible to transmission errors than i1 and q1.
Panasonic proposed modifying the constellation diagram from transmission to transmission such that the same bits map onto different points of the constellation diagrams, as a result of which the reliability of all the data bits will be substantially the same. FIG. 2 shows one particular proposal by Panasonic.
As shown in FIG. 2, the constellation diagram of FIG. 1 is modified such that i1 and q1 modulate the amplitudes of the carrier waves, while i2 and q2 modulate the phases. As a result, the reliabilities of i2 and q2 are improved, while those of i and q, are degraded. Thus, if the same data bits are retransmitted alternatively using the constellation diagrams shown in FIGS. 1 and 2, the data bits will have substantially the same reliability over time.
The constellation modification scheme proposed by Panasonic is intended to be implemented in both a transmitter and a receiver. FIG. 3(a) shows portions of a transmitter 300 for processing information bits representing user data and modulating a carrier wave with the processed information bits. In transmitter 300, a CRC adder 302 appends CRC bits to the information bits, a channel encoder 304 encodes the information bits appended with CRC bits to provide forward error correction, a channel interleaver 306 rearranges the encoded bits to protect against burst errors, a circuit block 308 punctures, or removes, certain ones of the encoded and interleaved bits to increase throughput, and a modulator 310 modulates the carrier wave with the encoded, interleaved, and punctured information bits. A controller 312 controls the channel encoding, interleaving, puncturing, and modulation processes. Controller 312 may act upon an acknowledgement signal or a negative acknowledgement signal received from the other side of the channel indicating that the previous transmission was successful or not.
With reference to FIG. 3(a), modification of the constellation diagram can be implemented in controller 312 and modulator 310, where controller 312 controls modulator 310 to modulate the carrier waves using the corresponding constellation diagram for each retransmission.
FIG. 3(b) shows portions of a receiver 400 for processing signals received from transmitter 300. In receiver 400, a demodulator 402 demodulates the received signals to generate the encoded, interleaved, and punctured information bits by removing the carrier wave. A de-interleaver 404 restores the order of the encoded bits by reversing the interleaving operation performed by channel interleaver 306. A combiner 406 combines multiple copies of data bits from the repeated transmissions to best estimate the data bits. A channel decoder 408 decodes the data bits using the forward error correction information added by channel encoder 304 to recover the data bits and the CRC bits. A CRC checker 410 checks the data bits and the CRC bits for any error. If CRC checker 410 detects an error, a negative acknowledgement (NACK) will be sent back to the transmitter to initiate a retransmission. Otherwise, an acknowledgement (ACK) will be sent. A buffer 412 buffers the received bits from previous transmissions so that the buffered bits can be combined with the bits received in subsequent transmissions of the same data bits by combiner 406. A controller 414 controls the operations of demodulator 402, de-interleaver 404, combiner 406, channel decoder 408, CRC checker 410, and buffer 412, and also controls the transmission of an acknowledgement or negative acknowledgement.
Similarly, with reference to FIG. 3(b), when a transmission arrives at receiver 400, demodulator 402 uses the corresponding constellation diagram, under the control of controller 414, to demodulate the received signals.
Modification of the constellation diagram can also be implemented through a bit rearranger, in which the bits within each symbol are rearranged, i.e., interleaved and/or inverted. As a result, although the same bit positions map onto the same positions on a constellation diagram, because the bits have changed positions and/or have been inverted, the bits can map onto different positions of the same constellation diagram from transmission to transmission. The result is the same as transmitting the bits in the symbol without interleaving or inversion, but using a different constellation diagram for each transmission. For example, equivalent to sending a symbol of 4 bits i1q1i2q2 using the constellation diagram shown in FIG. 2, the same 4 bits can be rearranged as i2q2ī1 q1, where ī1 and q1 are i1 and q1 logically inverted, respectively. A bit rearranger may be inserted between circuit block 308 and modulator 310 in FIG. 3(a). One example of a bit rearranger for implementing constellation diagram modification is discussed in U.S. patent application by Jae-Seung Yoom et al., published on Apr. 17, 2003, as Publication No. 2003/0072292.
There has also been proposed a method called subcarrier rearrangement for orthogonal frequency division multiplex (OFDM) systems to improve the performance of HARQ. In an OFDM system, a data bit stream is carried by a number of orthogonal frequency subcarriers. Because the subcarriers have different frequencies and experience different channel distortions, data bits transmitted over one subcarrier have different reliabilities than those over another subcarrier. The subcarrier rearrangement method addresses this lack of uniformity by swapping the subcarriers on retransmissions.
FIG. 4 shows a method of subcarrier rearrangement proposed by Kian Chung Beh, et al. in “Performance Evaluation of Hybrid ARQ Scheme of 3GPP LTE OFDMA System,” Proc. PIMRC 2007. In particular, user data are demultiplexed into N bit streams respectively carried by N subcarriers. In the first retransmission, the bit streams are shifted by N/2 subcarriers. In other words, the third quarter of the subcarriers now carry the bit streams originally transmitted over the first quarter of the subcarriers, and the fourth quarter of the subcarriers now carry the bit streams originally transmitted over the second quarter of the subcarriers, etc. For the second retransmission, the bit streams are further shifted by N/4 subcarriers. For the third retransmission, the bit streams are even further shifted by N/2 subcarriers. Then, the fourth retransmission, if required, will have the same arrangements as the original transmission.
The constellation modification, the bit rearranger implementing the constellation modification method, and the subcarrier rearrangement method all involve rearranging the bits or constellation diagram on a symbol basis. In other words, they all attempt to balance the reliabilities of the bits within the same symbols. However, bit reliabilities tend to fluctuate from symbol to symbol, and this fluctuation cannot be addressed by the methods proposed by Panasonic, Yoom et al., or Beh et al.